1. Technical Field
The present invention relates to a phosphor, a wavelength conversion element, a light source device, and a projector.
2. Related Art
JP-A-2009-277516 (Patent Document 1) describes a light source device utilizing fluorescence emitted from a phosphor as illumination light. This phosphor includes a binder composed of an organic material and phosphor particles dispersed in the binder. However, the heat resistance of the organic material is lower than that of an inorganic material, and therefore, when the binder is irradiated with high-intensity laser light as excitation light, the binder is degraded by heat.
JP-T-2013-518172 (Patent Document 2) discloses an inorganic phosphor which does not contain an organic material. The inorganic phosphor is obtained by sintering a plurality of phosphor particles.
However, the phosphor disclosed in Patent Document 2 has a plurality of voids, and therefore, the thermal conductivity of the phosphor is lower than the thermal conductivity of the phosphor particles. Due to this, when the phosphor is irradiated with high-intensity laser light as excitation light, heat is accumulated therein, and the luminous efficiency of the phosphor particles is deteriorated. Further, the more voids are included in the phosphor, the more easily the phosphor is broken, and therefore, there is a possibility that the phosphor is broken by thermal stress. For example, when the phosphor is irradiated with high-intensity excitation light, large thermal stress is generated by an in-plane temperature difference. Further, when excitation light is converged and irradiated on a narrow region so as to use the phosphor as a point light source, the temperature of the phosphor becomes high at the excitation light irradiation position, and therefore, large thermal stress is generated.
In the case where the phosphor having a plurality of voids is processed into a phosphor layer having a given thickness by, for example, polishing, the exposure of the voids on the surface of the phosphor layer cannot be avoided. Due to this, there exist concave portions derived from the voids on the surface of the phosphor layer. In the light source device described in Patent Document 1, in order to improve the utilization efficiency of light, an optical functional layer such as a dielectric multilayer film is sometimes provided on the surface of the phosphor layer. However, in the case where an optical functional layer is formed on the phosphor layer disclosed in Patent Document 2 having concave portions on the surface, it is difficult to form the optical functional layer in an intended state in the concave portions, and it is difficult to form the optical functional layer having intended properties.
On the other hand, the voids have a function to scatter fluorescence generated inside the phosphor. The light scattering function inside the phosphor is needed for increasing the extraction efficiency of fluorescence generated inside the phosphor and efficiently utilizing the fluorescence. When the voids are reduced, the scattering source in the phosphor is reduced, and therefore, not only the utilization efficiency of fluorescence is decreased, but also the fluorescence emission region is expanded, and the utilization efficiency of fluorescence by an optical system in the latter part is decreased.
Further, the improvement of the quantum yield of the phosphor has been demanded.